In insulin resistant states such as obesity and type 2 diabetes, expression of the insulin-responsive Glut4 glucose transporter is down-regulated in adipose tissue, but not in muscle. Glut4 expression is also down-regulated specifically in adipose tissue after fasting and this expression quickly recovers after refeeding. The physiologic significance of Glut4 regulation is uncertain as adipose tissue accounts for only a small fraction of insulin-stimulated glucose disposal. To study the physiologic significance of altered Glut4 expression in adipose tissue, the Kahn lab has developed transgenic models with constitutive overexpression or knockout of Glut4 selectively in adipose tissue. These models display reciprocal metabolic phenotypes with improved glucose tolerance as a result of Glut4 overexpression and whole-body insulin resistance as a result of Glut4 knockout. Notably, the adipose-specific overexpression of Glut4 (AG4OX) results in a mouse extremely susceptible to fasting-induced hypoglycemia. The physiologic mechanisms by which the adipose-specific modifications of Glut4 expression result in changes in systemic glucose homeostasis remain uncertain. Analysis of gene expression microarrays derived from adipose tissue from these transgenic models has revealed that overexpression or knockout of Glut4 results in reciprocal and coordinate changes in the expression of enzymes of branched-chain amino acid (BCAA) metabolism. Experiments have demonstrated that in the AG4OX mouse model, the coordinate down-regulation of BCAA enzyme expression is associated with increased levels of circulating BCAAs. BCAAs have previously been implicated as regulators of glucose homeostasis. The central aim of this proposal is to determine the impact of BCAAs on glucose homeostasis. Aim 1 will define physiologic mechanisms necessary for the maintenance of euglycemia during fasting. Aim 2 will determine whether alterations in BCAA metabolism cause changes in glucose homeostasis. Aim 3 investigates the mechanisms by which Glut4 overexpression results in altered BCAA metabolism. The proposal is designed to facilitate the training and career development of the applicant in the field of diabetes and metabolism. The career development plan includes the main research project, laboratory technique training, meetings and data presentation, and didactic seminars. The proposed research will take place in the Division of Endocrinology, Diabetes, and Metabolism at Beth Israel Deaconess Medical Center where all the resources to carry out the proposed research are readily available. The ultimate goal is for the applicant to become an independent investigator.